Ensure element exists before accessing dropdown

[saitho]

Description

Minor check if the toggle element can be found, before using it to get or create the dropdown instance.

Motivation & Context

We're using dropdowns in our main menu which slightly differs from the expectedBootstrap markup.
When closing the dropdown via Escape key, that triggers a JavaScript error as the element Bootstrap is looking for is not present in our code.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-{your_pr_number}--twbs-bootstrap.netlify.app/

Related issues

[julien-deramond]

Thanks for opening a PR, @saitho.

I haven’t reviewed the changes in detail yet, but just a quick heads-up:

• Please add unit tests. Ideally, they should reproduce the issue—failing before the fix and passing afterward.
• There’s a linting issue that needs to be resolved.
• If possible, could you also provide a minimal reproducible example (e.g., a small HTML snippet) that shows what wasn’t working on your end?